Explanation:

- The idiom "Keep in check" refers to maintaining control over something, preventing it from becoming excessive or out of control.

- Option 1: To keep something safe

- This suggests protection, but doesn’t imply control or moderation.

- Option 2: To place an order

- This means requesting something, unrelated to the idiom’s meaning.

- Option 3: To control something

- This is about regulation and ensuring things don’t get out of hand. This fits perfectly with “Keep in check.”

- Option 4: To test something

- Testing implies evaluation, not controlling.